In recent years, internal combustion engines have become known that use multiple types of fuel. Vehicles in which such internal combustion engines are mounted are known as flexible fuel vehicles (FFV), and known examples include vehicles that can use gasoline and ethanol, either alone or as blends.
As blended fuels of gasoline and ethanol, there are used ethanol-blended gasoline fuels with a wide range of compositions, from E3 (blend of approximately 3% ethanol with approximately 97% gasoline, by volume ratio) to E85 (blend of approximately 85% ethanol with approximately 15% gasoline, by volume ratio). Internal combustion engines for flexible fuel vehicles must carry out combustion optimized for such a wide range of compositions.
In this regard, since ethanol has different properties from gasoline, a means has been devised by which the ethanol-blended gasoline fuel is stored in the main fuel tank, and the ethanol-rich fuel or gasoline-rich fuel is separated by a fuel separator, the fuels being selectively supplied to the internal combustion engine based on the operating state of the internal combustion engine (Patent Document 1).
Because ethanol has a high boiling point and large latent heat of vaporization, it is difficult to form a combustible gaseous blend at the starting up of the internal combustion engine, and therefore ethanol has the drawback of a low start-up property for the internal combustion engine. On the other hand, ethanol has a high octane value, and therefore can minimize knocking in the high rotation/high load operational range of the internal combustion engine, and can allow the engine compression ratio to be increased.
The known methods of separating ethanol-blended gasoline fuels include separation by a polar separating membrane member, and especially separation by a pervaporation membrane. In separation using such a polar separating membrane member, the highly polar components such as ethanol preferentially permeate the membrane, and it is thereby possible to obtain fuel with enriched ethanol, for example, at the permeation side (Patent Documents 2 and 3).